(Reuters) - An unmanned European supply vessel carrying more than six tonnes of freight docked with the International Space Station (ISS) on Wednesday reinforcing Europe's role in the functioning of the ISS, space officials said.

European Space Agency (ESA) officials said the docking of Europe's third Automated Transfer Vehicle (ATV) was flawless when it eased into place without any intervention from astronauts in the space station.

They put the official docking time with the ISS at 2233 GMT and approximately 30 minutes later initial electrical connections to the ISS were confirmed.

Astronauts aboard the ISS will be able to enter the vessel after electric connections and seals keeping space atmosphere out of the station are checked.

"This rendez-vous and docking was the most critical phase," Jean-Jacques Dordain, director general of the ESA, said after the docking from a mission control centre in Toulouse, France.

"No other vehicle is able to do this kind of docking," Dordain said.

The vessel, dubbed "Edoardo Amaldi" after the Italian physicist and spaceflight pioneer, is the third ATV Europe has contributed to the ISS program.

The first docked with the space station in early 2008. A second docked early last year.

It was the first European mission to re-supply the ISS since the U.S. space shuttle fleet was retired last July.

Edoardo Amaldi was launched aboard an Ariane-5 rocket from ESA's launch centre in Kourou, French Guiana on the northeast coast of South America on March 23.

It will remain attached to the space station until August as astronauts remove its cargo and fill it with rubbish from the station.

It will then be thrust back toward earth, burning up on re-entry. Any remaining debris will be targeted to a remote area of the Pacific Ocean.

The ATV has more cargo capacity than Japan's HTV vessel also used to supply the ISS and over twice the capacity of a Russia's Progress vehicle.

American start-up SpaceX - brainchild of PayPal co-founder Elon Musk - has scheduled its first supply mission to the ISS aboard its Dragon spacecraft in late April.

The ATV will also be used as a 'space jack'. Residual gravity from the earth causes the space station to fall about 2.5 km (1.5 miles) a month. The vessel will ignite thrusters to lift the station back to a higher altitude.

ATV was developed by the ESA as part of a barter arrangement with the U.S. space agency NASA.

Instead of paying cash for its share of the station's operating costs and also to secure additional astronaut access, ESA is providing the ATV and other components.

A full ATV mission costs between 450 and 500 million euros ($585-650 million), the ATV spacecraft itself accounting for around 350 million Euros ($450 million), the ESA said.

The space station is a $150 billion project by 15 nations. Modular in design, most of the elements were transported aboard American space shuttles or Russian heavy-lift rockets. A final ISS element is scheduled to be delivered in late 2013 using a Russian Proton rocket.

China has so far not participated in the ISS preferring to concentrate on its own planned space station, though preliminary talks have indicated a possible change of policy.

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